As it is known, in electronic devices and equipment for wireless applications, such as, for example, those used in the mobile telephone field, transmission circuit blocks or transmitters are generally included and are so configured to generate signals to be transferred on an antenna at frequencies which are characteristic of the above-mentioned applications, typically in the radio frequency (RF). Particularly, each transmitter comprises a power amplifier connected to the antenna and is so configured as to generate and transfer voltage and current signals on the same antenna, having an appropriate width for such applications, i.e. to transfer suitable powers onto the antenna.
The increasing spread of battery-powered wireless devices of the portable type imposes that such power amplifiers ensure the transfer of the used power to the antenna, while ensuring a good conversion efficiency, i.e. a good ratio of the power transferred to the antenna to the power provided by the supply, for example, 30%. The power amplifiers require, among other things, low-consumption amplification stages.
An example of an integrated electronic amplifier of a known type that can be used in a transmitter is schematically shown in FIG. 5. Particularly, such amplifier 500 is a differential amplifier, comprising first diode-connected PMOS transistors M1 and M2, each of which being connected to a respective second PMOS transistor M3 or M4, to establish current mirrors. The first transistors M1 and M2 are mutually equal, and the second transistors M3 and M4 are mutually equal. It may be noted, particularly, that the differential amplifier 500 is adapted to control bias currents IM3,4 of the transistors M3 and M4, and the transfer on the load resistors RL of the differential input current signal iin=iin1−iin2, where the current signals iin1 and iin2 are schematically represented in FIG. 5 by current generators.
In greater detail, the bias currents IM1,2 relative to the transistors M1 and M2, and schematically illustrated by respective current generators, are reproduced on the transistors M3 and M4, approximately, based on the relationship:
                                          I                                          M                ⁢                                                                  ⁢                3                            ,              4                                            I                                          M                ⁢                                                                  ⁢                1                            ,              2                                      =                                                            (                                  W                  /                  L                                )                                                              M                  ⁢                                                                          ⁢                  3                                ,                4                                                                    (                                  W                  /                  L                                )                                                              M                  ⁢                                                                          ⁢                  1                                ,                2                                              =          N                                    (                  1          ⁢          a                )            in which W and L are the width and length of the transistors M1-M4 gate channel, while N is an integer known as a “mirror factor.” Based on formula 1a, the amplifier 500 allows controlling the currents in the transistors M3 and M4 thanks to the mirror. That is, once the bias currents have been set in M1 and M2, the bias currents in M3 and M4 are precisely established.
Furthermore, as it is known to those skilled in the art, the relationship in formula 1a is also valid for the signal currents. Particularly, by indicating the output differential current provided by the amplifier 500 on the loads RL with iout=id3−id4:
                                          i            out                                i                                          i                ⁢                                                                  ⁢                n                            ⁢                                                                                  =                                                            i                                  d                  ⁢                                                                          ⁢                  3                                            -                              i                                  d                  ⁢                                                                          ⁢                  4                                                                                    i                                  d                  ⁢                                                                          ⁢                  1                                            -                              d                                  d                  ⁢                                                                          ⁢                  2                                                              =                                                    gm                                  3                  ,                  4                                                            gm                                  1                  ,                  2                                                      =            N                                              (                  2          ⁢          a                )            where gm1,2 and gm3,4 are the transconductance values of the pairs of transistors M1, M2, and M3, M4, respectively. On the basis of formula 2a, the differential input signal iin of the amplifier 500 is amplified by an amount which is substantially equal to the mirror factor N. In other words, the output differential current iout on the output transistors M3 and M4 is amplified by the factor N.
It may be noted that, in order to ensure an amplification increase of the current signal iin, the mirror amplifier 500 requires an increase of the above-mentioned mirror factor N. However, this has the drawback of an increase in the bias currents IM3,4 of transistors M3 and M4 based on formula 1a, i.e. an increase of the static dissipation of such amplifier 500, which may be unacceptable for most applications of battery-powered portable electronic devices.